The use indefinite article modified by a relative clause

[3doubler]

Dear sir or ma'am,

I have a question about the use of indefinite article.
Please look into the below sentence.

In a Chicago that has a number of theaters, You can have far more chances to watch movies.

I was told that without 'a'-i.e. 'Chicago', not 'a Chicago'-the above sentence is not correct, because the noun modified by a relative clause should have 'a' or 'the'. I'd like to clarify about this concept. Please assume that we would talk, "Seoul that stands for the state-of-the-art IT techniques is the capital of South Korea". This sentence is not correct? Actually, It is difficult to understand that only a common noun can be modified by relative clauses except for a proper noun.

I am sincerely awaiting the help.
Thanks

[RonPurewal]

The given sentence is nonsense, since there is only one "Chicago".

The sentence about Seoul is also nonsense, for the same reason.

This...
In a city that has a large number of theaters, you can see a variety of movies
... makes sense.
In this sentence, we're restricting the discussion to cities that have a large number of theaters. (I.e., some cities don't; we're ignoring those cities.)

You could also write this:
In Chicago, which has a large number of theaters, you can see a variety of movies
Here, we're not narrowing anything down. (There's only one Chicago, so there's nothing to narrow down.)

[RonPurewal]

MUCH MORE IMPORTANTLY...

This whole principle"”modifiers without commas narrow things down, while modifiers with commas don't"”is very important in general English communication, but has never been tested on the GMAT. So, this whole thing is a non-issue in the first place.

[3doubler]

MUCH MORE IMPORTANTLY...

This whole principle"”modifiers without commas narrow things down, while modifiers with commas don't"”is very important in general English communication, but has never been tested on the GMAT. So, this whole thing is a non-issue in the first place.

Thank you for your comment, sir.

[RonPurewal]

Sure.